math
posted by raksha .
Suppose that the time (in hours) required to repair a machine is an exponentially distributed random variable with parameter λ (lambda) = 0.5.
What's the probability that a repair takes less than 5 hours? AND what's the conditional probability that a repair takes at least 11 hours, given that it takes more than 8 hours?

The probability distribution you are talking about is
f (x; lambda) = lambda exp^(lambda x), x > 0
It is discussed at http://en.wikipedia.org/wiki/Exponential_distribution
The probability that a repair takes less than x is
F (x; lambda) = 1  exp(lambda x)
For x = 1  exp(2.5) = 0.918
The fraction taking 8 hours or more to repair is
exp(4) = 0.01832
The fraction taking 11 hours to repair is
exp(5.5) = 0.00409
That means that 409/1832 = 22% of the repairs taking 8 hours or more require more than 11 hours.
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